When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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Monday, August 18, 2014

by Christa Marshall, E&E reporter, Climate Wire, August 18, 2014

Snowmageddon in Washington, D.C. Extreme floods in the United Kingdom last winter. A Texas heat wave two years ago.

For scientists, they all may be a byproduct of a warming Arctic. Or they might not, as much of the research on causation is still in early stages.

A new review article, released yesterday in Nature Geoscience, offers one additional theory about the link between the Arctic and extreme weather in mid-latitudes, pointing to a possible connection between snow in Siberia and unusual events in much of the United States, Europe and East Asia. But ultimately, the synthesis paper reports that much of the science remains uncertain about the link, as there is contradictory evidence for all of the main theories.

"The discussion on the topic until now has been 'he said, she said' where the ideas presented have been of individual scientists and have covered the spectrum of all possible opinions. This is the first that a group of leading experts with varying opinions all on the same paper and is the most authoritative consensus on the subject," said Judah Cohen, a scientist at Atmospheric and Environmental Research Inc. and lead author of the review article, which included scientists from six universities, research institutes and NOAA.

Basic statistics initially appear to reveal a link between Arctic changes and weather elsewhere. Since the late 1970s, Arctic September sea ice has declined at a rate of 12.4% per decade. The Arctic region has warmed about twice the global average, a phenomenon known as Arctic amplification. At the same time, there has been a trend toward many types of extreme weather in mid-latitudes.

The amount of precipitation on very wet days has increased since the late 1990s in mid-latitudes, for instance, and the percentage of warm days exceeding the 90th percentile has increased from 10% before 1980 to 16% now, the paper notes.

Dynamics that conflict with climate models

The scientists focused on winter weather extremes, as that season has "not always followed the warming script," Cohen said. "While global warming theory is consistent with record warm temperatures and more intense precipitation events, it does not directly explain cold extremes," the paper says. For example, the number of days continuously below freezing has increased in mid-latitudes, a dynamic that is not consistent with the projections of climate models.

The scientists examined dozens of papers and categorized them into three main theories about a possible link between the Arctic and different types of mid-latitude extreme weather. One suggests that warming spurs extra snow over Eurasia, as well as sea ice loss, which in turn shifts storm tracks south.

A second -- and highly discussed -- hypothesis is that a warming Arctic causes the jet stream to weaken and meander more in a North-South direction. Weather systems embedded in the jet stream then move more slowly, allowing events such as lengthy heat waves.

"The temperature difference between the Arctic and mid-latitudes is lessening. This is important because the west to east winds of the jet stream are driven by that temperature difference," said Rutgers University scientist Jennifer Francis, the lead proponent of the theory, in congressional testimony last year. Francis -- one of the co-authors of the new review article -- argued that the lessening temperature differential drives the North-South movement of air.

The third concept -- relevant for winter extremes -- is that the loss of sea ice in the Barents and Kara Seas in fall and winter helps create atmospheric "waves" that transfer energy from the low to the high atmosphere, that in turn alters the deep pressure known as the polar vortex, helping send Arctic blasts south.

The researchers added a fourth theory, that this "wave" phenomena may be boosted by increased snow cover over Siberia in October as well, in conjunction with the loss of sea ice. Previously, sea ice and snow cover on land largely had been talked about separately with the wave theory. "They can amplify the same result," Cohen said.

Four conflicting theories

Each of the theories has drawbacks, according to the paper. While most models show that warming leads to a southward movement of storm tracks, others do not, Cohen said.

The jet stream theory is probably the most controversial, as it is the least studied of the three, even as it is the most discussed, he said. The concept was the focal point of a National Academy of Sciences report in April synthesizing input from an earlier scientific meeting on the plausibility of the Arctic-weather link.

While the jet stream theory is plausible, there are also forces working against it "that may be canceling each other out," Cohen said. Arctic warming is occurring more at the surface and not at jet stream latitudes, he said. The tropics are more where warming is occurring at high latitudes, so it's not clear how much that is countering any influence on the jet stream from the north, he said.

For all three theories, it also is unclear how much Arctic amplification may be caused by mid-latitude weather in the first place via transfer of heat, as opposed to the reverse. Cohen called for more Arctic monitoring stations of everything from humidity to temperature to get a firmer grasp of what's happening.

Earlier this year, five climate scientists wrote a letter in Science during then-frigid temperatures in the eastern United States, urging the public to not make immediate conclusions about a climate change connection to the extreme weather. "The research linking summertime Arctic sea ice with wintertime climate over temperate latitudes deserves a fair hearing. But to make it the centerpiece of the public discourse on global warming is inappropriate and a distraction," the scientists wrote.

While the new analysis does not get any closer to certainty on any of the theories, it was important to get many of the lead scientists studying the topic in consensus on one paper, to provide an information baseline, Cohen said.

"I think the fact that anything was written is an accomplishment," he said.